This is one of the most important, difficult and misunderstood aspects of kime, focus of mental and physical energies at impact.
Many understand the idea of pressure and contraction, but somehow many people stop the breath and momentum at kime which stops the energy rather than impart it to opponent, and at same time create qio (chance).
Sensei Nishiyama used to constantly scold us: "Don't stop the momentum".
At kime we talk about pressure to floor and sharp contraction of total body musculature, and we say that for a moment nothing moves.
Of course, one cannot make pressure to floor and move at the same time, one need a firm base, stance, in order to apply pressure to floor.
We talk about all parts of the body peaking and stopping at once, so all parts peak and achieve maximum speed and momentum at the instant of impact. Sensei Nishiyama used to say that at kime even the eyes don’t blink, no wobble, any wobble means leak of energy.
At the same token, the whole purpose is maximize shocking power, moment of impact is when energy starts transferring to opponent, and sensei Nishiyama used to give us images such as a car driving at full speed, you put the breaks on but do not stop the engine, engine is at full power, or imagine you push a wall, you obviously are not moving but all muscles are contracting to direction of the push.
Or Sensei Nishiyama used to stand in front of me and Aiko San stood behind him, and he told me to punch at him and my body stops in front of him yet inside my body continue to hit Aiko San.
Or Aiko San used to stand in front of me and she wanted me to punch toward her, and she could easily tell if my momentum stopped or the energy went pass through her.
There are many elements involved in not stopping the energy, first, the intention and breath should be to infinity, focus at contact but not stop.
Than try to imagine that your body center hits the target, and your arm or foot are merely an extension, making direction and being tools of contact.
This is important because in many cases the momentum of the body does not match that of the technique, and in other cases there is a lot of isolated tension in the technique arm and leg, which limit the technique to top power, and in other cases someone tries to control the distance by the contraction of the arm or leg which stops and chokes the energy.
The control of distance at kime should be from the body center using the breath, breath makes kime, than stopping is not stopping the momentum and energy. A punch could be a once inch punch yet the mental extension is to infinity.
Harvey Panick, the golf master said the one should pour the energy of the swing into the ball, this is a great image for karate technique.
Without going to details, I like to mention that what was said about breathing will not be applicable if the stance is not used properly, for example, in a Gyaku Zuki, front stance at kime, pressure by breath is applied toward the front foot, and the reaction of this pressure comes back to the back foot, and from the back foot through the center energy transferred to the technique, so the more pressure toward front foot the stronger the back foot becomes, and more energy is transmitted to line of technique, a wheel of energy is being created.
Of course, this has to become reflexive, so all one has to think is intention and breath.
So the interaction between breathing, body center and both feet is essential for maximum transfer of energy and momentum.
I would also like to add an important drill that Aiko San used to do with me, she used to stand in front of me and without technique, only by using kiai, she demanded that I would go thorough her, and the breath had to come from the lower abdominals, we would do this for hours.
She kept on demanding that the throat will be like pipe without effort, otherwise top muscles will contract in access, and energy cannot flow through, the body has to be like a clear tunnel so energy can flow in the needed line and in the right amount.